We have now examined over 30 somatic cell hybrid clones from Chinese hamster and various mouse cells for the presence of endogenous xenotropic env related DNA sequences. It has been possible to correlate 15 of 28 xenotropic related HindIII restriction fragments from the BALB/c mouse with the presence or absence of various chromosomes. Of importance is the finding that the inducible Bxv-1 locus found in 5 different mouse strains can be assigned to an 18 kb fragment located at the distal end on chromosome 1 along with 2 other fragments (13 and 9 kb in size). A 10 kb fragment is located at the other end of chromosome 1. Of 11 other identifiable fragments, one each can be assigned to chromosomes 2, 4, 5, 12, 16, 17 and 19 while 2 each are located on chromosomes 7 and 15. Detailed studies have further shown that the majority of the xenotropic reactive fragments have internal restriction maps comparable to either known xenotropic proviruses or MCF precursor viral sequences. As part of this project we have used the same somatic cell hybrids to map the mouse homologues of various oncogenes. These include C-Harvey ras and C-fes to chromosome 7 and C-sis is to chromosome 15. Our long range goal is to study all xenotropic related sequences in the mouse with the aim of clarifying their role in oncogenesis.